Studies are proposed to establish T lymphocyte hybrids in order to evaluate the genetic control of characteristics that are unique or related to "T-ness." These hybrids, which will consist of both fibroblast-T lymphocyte and T lymphoma-T lymphocyte matings, will also be evaluated for T lymphocyte functions. In addition to routine hybridization of T lymphocytes with cell lines, microcell hybrids will be derived. A variety of different T lymphocyte markers will be used for the evaluation of the phenotype of the hybrids in addition to the testing of function. Attempts will be made to clone single cells taken from long-term lymphocyte cultures that have been derived following alloantigenic activation and maintained as dividing immunologically active cells in "conditioned medium," to characterize the clones functionally, and use members of these clones as the source of T lymphocytes contributing to the hybrids. A variety of techniques will be used to obtain variants of human lymphoblastoid cell lines that express only a single HLS-D haplotype; and mutagens will be used in an attempt to delete single genes from such haplotypes. These variants will be used to study the fine structure genetics of the HLA-D region with respect to the different phenotypic products that can be recognized as associated with that region either serologically or by cellular techniques. In addition the variants will be utilized in primed LD typing studies to improve the reagents that can be obtained for prospective donor-recipient matching by this test.